WO2006106944A1

WO2006106944A1 - Austenitic stainless steel

Info

Publication number: WO2006106944A1
Application number: PCT/JP2006/306894
Authority: WO
Inventors: Hirokazu Okada; Masaaki Igarashi; Kazuhiro Ogawa; Yasutaka Noguchi
Original assignee: Sumitomo Metal Industries, Ltd.
Priority date: 2005-04-04
Filing date: 2006-03-31
Publication date: 2006-10-12
Also published as: KR100931448B1; EP1867743A4; CN101151394A; CA2603681A1; JPWO2006106944A1; DK1867743T3; EP1867743B1; US7731895B2; JP4803174B2; EP1867743B9; CN100577844C; KR20070107166A; EP1867743A1; DK1867743T5; US20080089803A1; CA2603681C

Abstract

Disclosed is an austenitic stainless steel with high creep strength which is improved in creep ductility, weldability and hot workability. This steel contains, in mass%, 0.05-0.15% of C, not more than 2% of Si, 0.1-3% of Mn, 0.05-0.30% of P, not more than 0.03% of S, 15-28% of Cr, 8-55% of Ni, 0-3.0% of Cu, 0.05-0.6% of Ti, 0.001-0.5% of REM, 0.001-0.1% of sol. Al, not more than 0.03% of N and the balance of Fe and unavoidable impurities. This steel may further contain one or more elements selected from Mo, W, B, Nb, V, Co, Zr, Hf, Ta, Ca and Mg. It is desirable to use Nd as the REM.

Description

Technical field

TECHNICAL FIELD [0001] The present invention relates to an austenitic stainless steel excellent in high-temperature strength, which is used as a pipe and as a heat-resistant and pressure-resistant steel plate, steel bar, forged steel product, etc. in a power generation boiler engineering plant or the like.

Background art

Conventionally, austenitic stainless steels such as JIS SUS304H, SUS316H, SUS321H, SUS347H, and SUS310S have been used as equipment materials in boiler engineering plants that are used in high-temperature environments. It was. However, in recent years, the operating conditions of equipment in such a high temperature environment have become extremely severe, and as a result, the required performance for the materials used has become strict, and conventionally used austenitic stainless steel has an extremely insufficient high temperature strength. It becomes a situation to do! /

[0003] Precipitation of carbides is effective in improving the high-temperature strength, especially creep strength, of austenitic stainless steel, and the strengthening action of carbides such as MC, TiC, and NbC is beneficial.

23 6

It is used. In addition, the improvement of creep strength by using Cu-added copper is also used. It is also the force that the Cu phase that finely precipitates in the clave contributes to increasing the creep strength.

[0004] On the other hand, P, which is essentially an impurity element, contributes to the refinement of MC carbides and creep strength.

23 6

For example, in the invention disclosed in Patent Document 1, the creep strength is improved by adding P. However, increasing the amount of P deteriorates the weldability and creep ductility, so the amount added is limited. for that reason,

It can be said that the strengthening by P-adders is fully utilized!

Patent Document 1: Japanese Patent Application Laid-Open No. 62-243742

[0005] Patent Document 2 discloses an austenitic stainless steel containing more than 0.06% and up to 0.20% P. However, the steel was developed with the goal of improving high temperature salt resistance. For this reason, it contains a large amount of Si, from over 2.0% to 4.0%. Such a large amount of Si promotes precipitation of the σ phase, leading to deterioration of toughness and ductility. <o

Patent Document 2: Japanese Patent Laid-Open No. 7-118810

Disclosure of the invention

Problems to be solved by the invention

[0006] A first object of the present invention is to provide an austenitic stainless steel having high creep strength and excellent creep ductility and weldability.

A second object of the present invention is to provide an austenitic stainless steel that has improved hot workability in addition to the above characteristics.

Means for solving the problem

[0007] The present inventor attempted to improve creep ductility, weldability, and hot workability by adding trace elements in an austenitic stainless steel in which the P content was increased to increase the high-temperature strength. .

[0008] The present inventor has made various studies on elements that improve the creep ductility of austenitic stainless steel having a high P content. As a result, it was found that the addition of a very small amount of REM, especially Nd, dramatically improved creep ductility, and was also effective in improving weldability and hot workability.

[0009] Further, it was confirmed that when Ti is added in combination with P, not only the effect of refining carbide is obtained, but also the precipitation of phosphorus is caused during creep and the creep strength is improved.

[0010] Further, the effect of Cu addition was investigated with the aim of improving the creep strength. As a result, it was found that when the Cu content exceeds 3.0%, the effect of improving ductility by adding REM, especially Nd, is almost lost.

[0011] The present invention has been made on the basis of the above-mentioned findings, and the gist thereof lies in the following austenitic stainless steels (1) to (4).

In [0012] (1) Weight ^{_{0/0, C:. 0.}} 05~0 15%, Si: 2% or less, Mn: 0. 1~3%, P : 0. 05~0.

30%, S: 0.03% or less, Cr: 15-28%, Ni: 8-55%, Cu: 0-3.0%, Ti: 0.05-0.6%, REM: 0.001 Austenitic stainless steel containing ~ 0.5%, sol.Al: 0.001 ~ 0.1%, N: 0.03% or less, the balance being Fe and inevitable impurity power.

[0013] (2) instead of a part of Fe, by mass ^_0/0, further Mo: 0. 05~5%, W: 0. 05~10%, was However, Mo + (WZ2) is 5% or less, B: 0. 0005 to 0.05%, Nb: 0.05 to 0.8%, V: 0.02 to: L 5%, Co: 0.05 to 5 %, Zr: 0.005 to 0.2%, Hf: 0.0005 to 1%, and Ta: 0.01 to 8%, and the austenitic stainless steel according to (1) above. steel.

[0014] (3) instead of a part of Fe, by mass ^_0/0, further Mg:. 0. 0,005-0 0.05% and Ca: containing one or both of 0.0 005-0 0.05%. The austenitic stainless steel according to (1) or (2) above.

[0015] (4) Austenitic stainless ίoka as described in any one of (1) to (3) above, wherein REM is Nd.

REM is an abbreviation for rare earth element, and is a collective term for 17 elements with 15 lanthanoid elements and Sc and Y.

[0016] The stainless steel of the present invention can be widely applied to applications requiring high-temperature strength and corrosion resistance, such as steel pipes, steel plates, steel bars, steel products, and forged products.

BEST MODE FOR CARRYING OUT THE INVENTION

[0017] The reason for limiting the component range will be described below. The% regarding the component content is “mass%”.

C: 0.05-0.15%

C is an effective and important element for securing the tensile strength and creep strength required when used in a high temperature environment. In the steel of the present invention, unless the content is 0.05% or more, the above-described effects cannot be exhibited and the target high-temperature strength cannot be obtained. However, even if the content exceeds 0.15%, only the amount of undissolved carbide in the solution state increases, and it does not contribute to the improvement of the high temperature strength. It also degrades mechanical properties such as toughness and weldability. Therefore, the C content is set to 0.05 to 0.15%. A more preferred upper limit is 0.13%. A more preferred upper limit is 0.12%.

[0018] Si: 2% or less

Si is added as a deoxidizing element, and is an element effective for enhancing oxidation resistance and steam oxidation resistance. In order to obtain the effect, 0.1% or more is desirable. However, if its content exceeds 2%, precipitation of intermetallic compound phases such as σ phase is promoted, and toughness and ductility are reduced due to deterioration of structural stability at high temperatures. Also melt The contact property and hot workability also deteriorate. Therefore, the Si content is set to 2% or less. Preferred is 1% or less.

[0019] Mn: 0.1-3%

Mn has a deoxidizing effect on molten steel, similar to Si, and fixes S, which is inevitably contained in the steel, as a sulfide and improves hot workability. In order to obtain the effect sufficiently, it is necessary to contain 0.1% or more. However, if its content exceeds 3%, precipitation of intermetallic compound phases such as the σ phase is promoted, and the structural stability, high temperature strength, and mechanical properties deteriorate. The force S and the Mn content were 0.1 to 3%. More preferred lower and upper limits are 0.2 and 2%, respectively. A more preferred upper limit is 1.5%.

[0020] P: 0.05-0.30%

P precipitates fine precipitates of carbides and phosphorus precipitates with Ti and Fe, and improves the tap strength of the steel of the present invention. In order to obtain this effect, a content of 0.05% or more is necessary. Usually, the additive of P is accompanied by deterioration of creep ductility, weldability and hot workability, but in the steel of the present invention, the deterioration of the above characteristics is suppressed by the additive of REM. However, if P is added excessively, the effect of REM additive, especially the effect of Nd additive, is lost, so its content must be 0.3% or less. Therefore, the appropriate P content is 0.05 to 0.3%. More preferable lower limit and upper limit are 0.06% and 0.25%, respectively, more preferable lower limit is an amount exceeding 0.08%, and further preferable upper limit is 0.20%.

[0021] S: 0.03% or less

S is contained as an inevitable impurity in the steel, so that hot workability is remarkably lowered. The smaller the content, the better.

[0022] Cr: 15-28%

Cr is an important element for ensuring oxidation resistance, steam oxidation resistance, high-temperature corrosion resistance, and the like, and further contributes to the formation of Cr-based carbides and increased strength. Therefore, it is made to contain 15% or more. The higher the Cr content, the better the corrosion resistance. When the content exceeds 28%, the austenite structure becomes unstable, and it is easy to produce intermetallic compounds such as σ phase and α-Cr phase. . Therefore, the Cr content should be 15-28%. More preferable lower and upper limits are 16% and 25%, respectively. Further preferred under The upper limit is 17%, and a more preferable upper limit is 23%.

[0023] Ni: 8-55%

Ni is an essential element for securing a stable austenite structure. The minimum required content is determined by the content of ferrite-forming elements such as Cr, Mo, W, and Nb and austenite-generating elements such as C and N contained in the steel. Strength required to contain 15% or more of Cr in the steel of the present invention If Ni is less than 8% with respect to the amount of Cr, it is difficult to form an austenite single phase structure. In addition, the austenite structure becomes unstable with long-term use at high temperatures, and high temperature strength and toughness deteriorate significantly due to embrittlement phase precipitation such as σ phase, so that it can withstand use as a heat and pressure resistant member. Can not. Even if the content exceeds 55%, the effect is saturated and the economic efficiency is impaired. Therefore, the Ni content is 8 to 55%. A more preferred upper limit is 25%, and a more preferred upper limit is 15%.

[0024] Cu: 0 to 3.0%

Cu is one of the elements that precipitates in the austenite matrix as a fine Cu phase during the use of the steel of the present invention at a high temperature and greatly improves the creep strength. Therefore, when such an effect is desired, Cu can be contained. However, excessive Cu content degrades hot workability and creep ductility. In the steel of the present invention, when the Cu content exceeds 3.0%, the effect of improving the creep ductility by adding REM described later decreases. Therefore, the Cu content of the steel of the present invention is set to 0 to 3.0%. A more preferred upper limit is 2.0%, and a still more preferred upper limit is 0.9%. In the present invention, Cu may not be contained, but when it is contained in order to obtain the effect of improving the creep strength, the lower limit of the content is preferably 0.01%.

[0025] Ti: 0.05-0.6%

Ti forms carbides and contributes to improving high temperature strength. In the steel of the present invention, it is precipitated as phosphorus deposits by complex addition with P and further contributes to the improvement of creep strength. If its content is less than 0.05%, a sufficient effect cannot be obtained, and if it exceeds 0.6%, weldability and hot workability are deteriorated. Therefore, the appropriate Ti content is 0.05-0.6%. More preferred lower and upper limits are 0.06% and 0.5%, respectively.

[0026] sol.Al: 0.001 to 0.1% In the steel of the present invention, Al whose content is a problem is sol.Al (acid-soluble Al). A1 is an element added as a deoxidizer for molten steel, and in order to exert its effect, it is necessary to contain 0.001% or more as sol. A1. However, if its content exceeds 0.1%, it promotes precipitation of intermetallic compounds such as σ phase during use at high temperature, and lowers toughness, ductility and high temperature strength. Therefore, the appropriate range of sol.Al content is 0.001-0. 1%. More preferred lower and upper limits are 0.005% and 0.05%, respectively. Further preferred lower and upper limits are 0.01% and 0.03%, respectively.

[0027] N: 0.03% or less

In the steel of the present invention containing Ti, when the N content exceeds 0.03%, Ti N precipitates at a high temperature, which remains in the steel as coarse undissolved nitride and is hot-worked. Decrease cold workability and cold workability. Therefore, the N content should be 0.03% or less. The smaller the N content is, the more desirable it is, more preferably 0.02% or less, and even more preferably 0.015% or less.

[0028] REM: 0.001 to 0.5%

In the steel of the present invention, REM is one of important elements. By adding REM, the creep ductility, weldability, and hot workability deteriorated by the high concentration P additive can be recovered. In order to exert the effect, it is necessary to contain 0.001% or more. However, when its content exceeds 0.5%, inclusions such as acid inclusions increase. Therefore, the proper range of REM content is 0.001 to 0.5%. More preferable lower and upper limits are 0.005% and 0.2%, respectively, and a more preferable upper limit is less than 0.1%.

The elements in REM can be added as a mixture, such as mish metal, which can be added alone. Of particular interest in REM! / Is Nd.

[0029] One of the steels of the present invention is an austenitic stainless steel in which, in addition to the above components, the balance is Fe and impurities. Another steel of the present invention is an austenitic system containing one or more selected from Mo, W, B, Nb, V, Co, Zr, Hf and Ta in order to further improve the high temperature strength. Stainless steel. Hereinafter, these components will be described.

[0030] Mo: 0.05 to 5%, W: 0.05 to 10%, but less than 5% with Mo + (WZ2) Mo and W are not essential components of the steel of the present invention. However, since these are effective elements for improving the high temperature strength and creep strength, they can be added as necessary. In the case of single addition, the content should be 0.05% or more, respectively. In the case of complex addition, the total should be 0.05% or more. However, if Mo exceeds 5% and W exceeds 10%, the strength improvement effect is saturated and the formation of intermetallic compounds such as the σ phase is caused, and the structural stability and hot workability deteriorate. Therefore, the upper limit for addition is preferably 5% for Mo alone, 10% for W alone, and 5% or less for Mo + (WZ2) when Mo and W are added in combination. Since W is a ferrite-forming element, the W content is more preferably less than 4% in order to stabilize the austenite structure.

[0031] B: 0.005% to 0.05%

B exists in the grain boundary in carbonitride or by itself, promotes fine dispersion precipitation of carbonitride during use at high temperature, and suppresses intergranular slip by strengthening the grain boundary, thereby increasing high temperature strength and creep strength. To improve. In order to exert its effect, it is necessary to contain 0.0005% or more, but if it exceeds 0.05%, the weldability deteriorates. Therefore, the appropriate range of B content when added is 0.0005-0.05%. More preferred lower and upper limits are 0.001% and 0.01%, respectively. A more preferred upper limit is 0.005%.

[0032] Nb: 0.05-0.8%

Nb also forms carbonitrides like Ti and improves creep rupture strength. If its content is less than 0.05%, a sufficient effect cannot be obtained, and if it exceeds 0.8%, hot workability, in addition to deterioration of mechanical properties due to increase in weldability and undissolved nitride, In particular, the high hot ductility at 1200 ° C or higher is significantly reduced. Therefore, the appropriate Nb content is 0.05-0.8%. A more preferred upper limit is 0.6%.

[0033] V: 0.02 ~: L. 5%

V is an element that forms carbides and is effective in improving high temperature strength and creep strength. When added, the effect is not good if its content is less than 0.02%. If it exceeds 1.5%, the hot corrosion resistance deteriorates, and ductility and toughness deteriorate due to brittle soot phase precipitation. Therefore, the appropriate V content is 0.02 ~: L 5%. More preferred lower and upper limits are 0.04% and 1%, respectively. [0034] Co: 0.05--5%

Co, like Ni, stabilizes the austenite structure and contributes to improved creep strength. If the content is less than 0.05%, the effect is not sufficient. If the content exceeds 5%, the effect is saturated and the economic efficiency is lowered. Therefore, the Co content when added should be 0.05 to 5%.

[0035] Zr: 0.0005% to 0.2%

Zr contributes to grain boundary strengthening and improves high temperature strength and creep strength. Furthermore, it has the effect of improving the hot workability by fixing S. In order to bring out the effect, the mechanical properties such as ductility and toughness are deteriorated when the force exceeds 0.205%, which is required to contain 0.0005% or more. Therefore, the appropriate Zr content when added is 0.0005-0. 2%. A more preferable lower limit and upper limit are 0.01% and 0.1%, respectively, and a more preferable upper limit is 0.05%.

[0036] Hf: 0.0005% to 1%

Hf mainly contributes to grain boundary strengthening and improves creep strength. If the content is less than 0.0 005%, there is no effect. On the other hand, if its content exceeds 1%, the caulking property and weldability are impaired. Therefore, when Zr is added, the appropriate content is 0.0005 to 1%. More preferred lower and upper limits are 0.01% and 0.8%, respectively, and more preferred lower and upper limits are 0.02% and 0.5%, respectively.

[0037] Ta: 0.01-8%

Ta forms carbonitride and improves the high temperature strength and creep strength as a solid solution strengthening element. If the content is less than 0.01%, there is no effect. On the other hand, if the Ta content exceeds 8%, the cache properties and mechanical properties are impaired. Therefore, when Ta is added, its content should be 0.01 to 8%. More preferred lower and upper limits are 0.1% and 7%, respectively, and more preferred lower and upper limits are 0.5% and 6%, respectively.

[0038] Yet another steel of the present invention is an austenitic stainless steel further containing one or both of Ca and Mg in addition to the above components. As described below, Ca and Mg improve the hot workability of the steel of the present invention.

[0039] Mg and Ca: 0.005% to 0.05% each

Mg and Ca improve the hot workability by adhering S, which inhibits hot workability, as sulfides. Be good. If each content is less than 0.0005%, there is no effect. On the other hand, Mg and Ca in a content exceeding 0.05% each harm the steel quality and reduce hot workability and ductility. Therefore, the content of Mg and Ca when added is preferably 0.005-0.05%. More preferred lower and upper limits are 0.001% and 0.02%, respectively. A more preferred upper limit is 0.01%.

[0040] When producing the steel of the present invention, the following method is recommended.

First, a steel ingot having the above-mentioned chemical composition is produced by a normal stainless steel melting and forging method. The obtained steel ingot is forged or formed into billets by forging or split rolling, and then subjected to hot working such as hot extrusion or hot rolling. The heating temperature before hot working is preferably 1160 ° C or higher and 1250 ° C or lower. The hot working finish temperature is preferably 1150 ° C or higher. In addition, after processing is completed, to prevent precipitation of coarse carbonitrides, it is recommended to cool at a cooling rate of at least 0.25 ° CZ seconds or more until at least 500 ° C! /.

[0041] After the hot working, a final heat treatment may be performed, or a cold working may be added if necessary. Before cold working, it is necessary to dissolve carbonitride by intermediate heat treatment, and this intermediate heat treatment is performed at a temperature higher than the lower one of the heating temperature before hot working or the end temperature of hot working. Good.

[0042] In cold working, it is preferable to apply a strain of 10% or more. Two or more cold workings may be applied. The heat treatment of the final product is preferably carried out at a temperature in the range of 1170 to 1300 ° C, at a temperature higher than the hot working finish temperature or 10 ° C above the intermediate heat treatment temperature mentioned above! In order to suppress the precipitation of coarse carbonitrides, it is recommended to cool at a cooling rate as fast as 0.25 ° CZ seconds or more even after the final heat treatment.

Example

[0043] Steel having the chemical composition shown in Table 1 was melted in a high-frequency vacuum melting furnace to obtain a 30 kg ingot having an outer diameter of 120 mm. Steels Nos. 1 to 19 in the table are invention steels and A to F are comparative steels.

The obtained ingot was hot forged into a 40 mm thick plate, and a round bar tensile test piece (diameter 10 mm, length 130 mm) for evaluating high temperature ductility was produced by machining. Further, a plate material having a thickness of 15 mm was formed by hot forging, and after softening heat treatment, it was cold-rolled to a thickness of 10 mm, held at 1150 ° C. for 15 minutes, and then water-cooled. [0044] Creep test pieces and ballast train test pieces were produced by the above plate material force machining. The shape of the creep test piece is a round bar test piece with a diameter of 6 mm and a distance between the gauge points of 30 mm, and the Valestrain test piece is a plate-like test piece with a thickness of 4 mm, a width of 100 mm and a length of 100 mm.

[0045] For the evaluation of ductility at high temperature, using the above-mentioned test piece for high temperature ductility evaluation, heating to 1220 ° C and holding for 3 minutes, conducting a high-speed tensile test with a strain rate of 5Zs, The squeezing rate was obtained from the cross section. It has been found that if the drawing rate is 60% or more at that temperature, no major problem occurs in hot working such as hot extrusion. Therefore, steel with a drawing ratio of 60% or more was selected as steel with good hot workability.

[0046] Using the above-mentioned creep rupture test piece, a tape rupture test was performed in the atmosphere of 700 ° C under a stress of 147 MPa to obtain a rupture life and a squeeze squeeze. The creep ductility was evaluated from the drawing at break.

[0047] The ballast train test for evaluating weldability was performed by the TIG method with a heat input of 19 kjZcm and an applied strain of 1.5%, and the weldability was evaluated from the total crack length.

Table 2 shows the results of the above tests.

[0048] [Table 1]

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[0050] In the comparative steels A, B and C, the P content is changed. For stainless steel pipes used for boiler heat exchange, for example, as specified in JIS G3463, P is limited to 0.040% or less. Therefore, comparative steel A corresponds to the P content of general stainless steel. As shown in Table 2, the creep rupture life is improved by increasing the amount of P added, but the fracture drawing, weldability and hot ductility are significantly reduced.

[0051] Steels No. 1 to No. 4 and No. 19 that are the steels of the present invention are steels that have been improved in creep rupture life by adding P in the same manner as comparative steels B and C. In these steels, due to the addition of Nd or La and Ce, there was no decrease in creep ductility, weldability and hot ductility as seen in comparative steels. This is an improvement over comparative steel A, which is a general level.

[0052] Comparative steel D is inferior in creep characteristics because it does not contain Ti and does not contain Ti, which does not contain Ti and contains P and Nd equivalent to steel symbol 2 of the steel of the present invention. Steel symbols 5 and 6 are obtained by further adding Cu to increase the creep strength. Comparative Steel E contains more than 3.0% Cu As shown here, the addition of excess Cu loses the creep ductility, weldability, and hot ductility improvement effects of Nd additive. This also shows that the Cu content needs to be 3.0% or less.

As described above, the steel of the present invention can further contain one or more of W, Mo, B, Nb, V, Co, Zr, Hf, Ta, Mg and Ca. As indicated by steel symbols 7-18, the addition of these elements further improves high temperature ductility and creep rupture strength.

Industrial applicability

[0054] The austenitic stainless steel of the present invention is a steel in which P and REM, particularly Nd, are added in combination, and the hot workability is remarkably improved in addition to having a large high-temperature strength. Furthermore, improved toughness on the high temperature and long time side has also been achieved.

The steel of the present invention is suitable as a heat and pressure resistant member used at a high temperature of 650 ° C to 700 ° C or higher. Since the plant using this steel can operate with high efficiency, the manufacturing cost of the product manufactured with the plant can be reduced.

Claims

The scope of the claims

[1] in a weight ^{_{0/0, C:. 0.05~0}} 15%, Si: 2% or less, Mn: 0.1~3%, P: 0.05~0.30%, S: 0.03% or less, Cr: 15 to 28% , Ni: 8-55%, Cu: 0-3.0%, Ti: 0.05-0.6%, REM: 0.001-0.5%, sol.Al: 0.001-0.1%, N: 0.03% or less, the balance Austenitic stainless steel in which Fe and unavoidable impurity powers.

[2] instead of a part of Fe, by mass ^_0/0, further Mo: 0.05~5%, W: 0.05~10 %, provided that

Mo + (WZ2) is 5% or less, B: 0.0005-0.05%, Nb: 0.05-0.8%, V: 0.02-1.5%, Co: 0.05-5%, Zr: 0.0005-0.2%, Hf: 0.0005-1% 2. The austenitic stainless steel according to claim 1, containing at least one of Ta and 0.01 to 8%.

[3] instead of a part of Fe, by mass ^_0/0, further Mg: 0.0005 to 0.05% and Ca: 0.000

The austenitic stainless steel according to claim 1 or 2, which contains one or both of 5 to 0.05%.

[4] The austenitic stainless steel ίoka according to any one of claims 1 to 3, wherein REM is Nd.